Method and apparatus for transmitting precoding martrix index and performing precoding

ABSTRACT

A method and an apparatus for sending a Precoding Matrix Index (PMI) and performing precoding are provided in the embodiments of the present invention. The method for sending the PMI comprises the following steps: a user equipment acquires the transmission channel capability of carrying the PMI; according to the transmission channel capability of carrying the PMI, the precoding matrices are selected from a locally-stored first codebook set to form a second codebook set; a first precoding matrix is selected from the second codebook set; an index corresponding to the first precoding matrix is sent to a base station over the transmission channel so as to make the base station can find out the first precoding matrix according to the index and precode the data according to the first precoding matrix. The embodiments of the present invention can realize the flexible configuration and use of the PMI.

CROSS-REFERENCE

This application is a continuation of International Application No.PCT/CN2011/073632, filed on May 4, 2011, which claims priority toChinese application No. 201010169491.4, entitled “A METHOD AND APPARATUSFOR SENDING PMI AND PERFORMING PRECODING,” filed on May 4, 2010, whichis incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to the field of communication technology,and more specifically, to a method and apparatus for PMI (Precodingmatrix Index) transmission and precoding.

DESCRIPTION OF THE RELATED ART

In 3GPP (3^(rd) Generation Partnership Project) LTE (Long TermEvolution) R10, eight antennas are introduced into a BS (Base Station).There are two configurations for antenna arrangement: a ULA antennaconfiguration as shown in FIG. 1, and a bipolarization antennaconfiguration as shown in FIG. 2. With respect to discussions aboutcurrent 8-antenna codebooks in 3GPP, there is an agreement as follows.Current precoding is composed of two sections:W=W ₁ W ₂  (1)

wherein, W₁ belongs to a codebook set C₁, W₂ belongs to another codebookset C₂. W₁ mainly represents a wideband/long-term channelcharacteristic; W₂ represents a subband/short-term channelcharacteristic.

In 3GPP LTE R8, PMI (Precoding Matrix Index) information can be reportedover PUCCH (Physical Uplink control CHannel) or PUSCH (Physical UplinkShared CHannel), wherein PMI information includes at most 4 bits.

In the prior art, it is unable to achieve flexible configuration orutilization of PMI according to transmission channel conditions.

SUMMARY OF THE INVENTION

A method for transmitting PMI is provided in an aspect of thisinvention, capable of achieving flexible configuration and utilizationof PMI, includes: selecting a first precoding matrix from a secondcodebook set by a user equipment (UE), wherein the second codebook setis set up according to a capability of a transmission channel to carryPMI; transmitting, by the UE, an index item corresponding to the firstprecoding matrix to a base station (BS) over the transmission channel,so as to enable the BS to search for the first precoding matrixaccording to the index item and perform precoding on data according tothe first precoding matrix.

A method for precoding capable of achieving flexible configuration andutilization of PMI is provided in another aspect of this invention,includes: receiving, by a base station (BS), an index item correspondingto a first precoding matrix transmitted by a user equipment (UE) over atransmission channel, wherein the first precoding matrix is selectedfrom a second codebook set by the UE, and the second codebook set is setup according to the capability of the transmission channel to carry PMI;searching for, by the BS, the first precoding matrix in a third codebookset locally stored according to the index item, and performing precodingon data according to the first precoding matrix, wherein the thirdcodebook set is identical to the second codebook set.

A UE capable of achieving flexible configuration and utilization of PMIis provided in still another aspect of this invention, includes: aselecting unit, configured to select a first precoding matrix from asecond codebook set, wherein the second codebook set is set up accordingto a capability of a transmission channel to carry PMI; a transmittingunit, configured to transmit an index item corresponding to the firstprecoding matrix to a base station (BS) over the transmission channel,so as to enable the BS to search for the first precoding matrixaccording to the index item, and perform precoding on data according tothe first precoding matrix.

A BS is further provided according to another aspect of this invention,includes: a receiving unit, configured to receive an index itemcorresponding to a first precoding matrix transmitted by a userequipment (UE) over a transmission channel, wherein the first precodingmatrix is selected from a second codebook set by the UE, and the secondcodebook set is set up according to a capability of a transmissionchannel to carry PMI; a precoding unit, configured to search for thefirst precoding matrix in a third codebook set locally stored by the BSaccording to the index item received by the receiving unit, andperforming precoding on data according to the first precoding matrix,wherein the third codebook set is identical to the second codebook set.

In view of above aspects of this invention, precoding matrixes havingdifference accuracies can be selected according to the capability of thetransmission channel to carry PMI to perform precoding on datatransmitted from the BS to the UE, so as to achieve flexibleconfiguration and utilization of PMI.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural diagram of a ULA antenna in the priorart;

FIG. 2 is a schematic structural diagram of a bipolarization antenna inthe prior art;

FIG. 3 is a flowchart of a method for transmitting PMI according to anembodiment of this invention;

FIG. 4 is a flowchart of a precoding method according to an embodimentof this invention;

FIG. 5 is a schematic structural diagram of a UE according to anembodiment of this invention;

FIG. 6 is a schematic structural diagram of a BS according to anembodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A clear, complete description will be given to technical solutions ofembodiments of this invention in connection with accompanying drawingsof embodiments of this invention. Obviously, embodiments describedherein are merely some embodiments of this invention, but not all ofthem. Based on those embodiments of this invention, other embodimentscan occur to those skilled in the art without any creative efforts, allof which fall within the scope of this invention.

FIG. 3 is a flowchart of a method for transmitting PMI according to anembodiment of this invention, including the following steps.

Step 301. A UE acquires a capability of a transmission channel to carryPMI.

In this embodiment, the transmission channel refers to a channelcarrying or bearing PMI, for example, including: a PUCCH channel, aPUSCH channel, etc. The way to acquire the capability of thetransmission channel to carry PMI may include: receiving a signaling fornotifying PMI carrying capability of the transmission channel sent froma BS, or acquiring from a locally stored list of the capability of thetransmission channel to carry PMI. The capability of a transmissionchannel to carry PMI refers to the maximum number of bits for carryingPMI on the transmission channel, for example, on a PUCCH channel at most4 bits for carrying PMI, on a PUSCH channel at most 6 bits for carryingPMI. Such capability can be preconfigured according to data transmissionrequirements.

Optionally, at step 302, according to the capability of the transmissionchannel to carry PMI, the UE can select a precoding matrix from a firstcodebook set locally stored to form a second codebook set.

Particularly, all or a subset of the first codebook set can be selectedas the second codebook set according to actual transmission channelconditions. The first codebook set may be a set of all codebooks usedfor precoding in UE data transmission and locally stored by the BS orthe UE, separately.

The precoding matrix of the first codebook set may be in the form asfollows.

$\begin{matrix}{W = {\underset{\underset{w_{1}}{︸}}{\begin{pmatrix}{DA} & 0 \\0 & {{DA}\;\Lambda}\end{pmatrix}}\underset{w_{2}}{\begin{pmatrix}I \\X\end{pmatrix}}}} & (2)\end{matrix}$

In equation (2), assume the number of transmitting antennas isN_(t)(N_(t) is an even), the rank of the matrix is R, W₁ is a N_(t)×2Rmatrix, W₂ is a 2R×R matrix. I is a R×R identity matrix, X is a R×Rdiagonal matrix, D is a N_(t)/2×N_(t)/2 diagonal matrix, A is aN_(t)/2×R matrix, A is a R×R diagonal matrix. Wherein,

$\begin{matrix}{\Lambda = {{\begin{bmatrix}a_{1} & 0 & 0 \\0 & \ddots & 0 \\0 & 0 & a_{R}\end{bmatrix}\mspace{14mu} X} = \begin{bmatrix}\beta_{1} & 0 & 0 \\0 & \ddots & 0 \\0 & 0 & \beta_{R}\end{bmatrix}}} & (3)\end{matrix}$

As to the bipolarization antennas shown in FIG. 2, Λ and X are used toadjust the phase relationship between two sets of polarized antennas.α_(i),β_(i) (i=1, . . . R) are scalars having modulus of 1.

Taking 8 transmit antennas and a precoding matrix having rank of 1 as anexample, in the arrangement shown in FIG. 2, D is a 4*4 diagonal matrix,which can be represented with b bits, there are T=2^(b) possible values.

$\begin{matrix}{{D \in \{ \begin{pmatrix}1 & 0 & 0 & 0 \\0 & {\mathbb{e}}^{j\;\pi\;{g/8}} & 0 & 0 \\0 & 0 & {\mathbb{e}}^{2j\;\pi\;{g/8}} & 0 \\0 & 0 & 0 & {\mathbb{e}}^{3j\;\pi\;{g/8}}\end{pmatrix} \}},( {{g \in 0},{{\ldots\; T} - 1}} )} & (4)\end{matrix}$

A can be obtained from an existing DFT (Discrete Fourier Transform)based codebook structure through the following equation.

$\begin{matrix}{{e_{m}^{(g)} = {\frac{1}{\sqrt{M}}\lbrack {w_{0m}^{(g)}\mspace{14mu}\ldots\mspace{14mu} w_{{({M - 1})}m}^{(g)}} \rbrack}^{T}}{w_{nm}^{(g)} = {\exp\{ {j\frac{2\pi\; n}{M}( {m + \frac{g}{G}} )} \}}}} & (5)\end{matrix}$

Wherein, M is the number of DFT dimensions, m=0, 1, . . . M−1; n=0, 1 .. . M−1. As to DFT corresponding to 4 antennas, M=4. G is the number ofgroups of DFT, g=0, 1, . . . , G−1. e_(m) ^((g)) is a precoding vectorin the codebook set, w_(nm) ^((g)) represents various elements in e_(m)^((g)). For instance, in the case of G=2, four 4×1 precoding vectors canbe obtained from each 4-dimensional DFT structure, and two DFTstructures may produce eight 4×1 precoding vectors. Hence, if Λ isrepresented with 3 bits, the values of A can be obtained from the eight4×1 precoding vectors. A is a 1×1 matrix with values of points on a unitcircle, and represented with b bits, for example. Thus, there are totalT=2^(b) possible values and its value can be e^(j2πk/T) (k=0, . . . ,T−1). For example, with a 1-bit representation, it has values (1, −1). Aand X are both 1×1 matrix with their values of points on a unit circleand represented with b bits, for example. Thus, there are total T=2^(b)possible values and their value can be e^(j2πk/T) (k=0, . . . , T−1).

Suppose that the first codebook set is represented with 7 bits. Becausethe function of D, A is to adjust the direction of beam transmission,they are totally represented with 5 bits. Assume that D is representedwith 1 bit, in equation (4), g is set to 1 or −1 constantly.

$\begin{matrix}{D \in \{ {\begin{pmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{pmatrix},\begin{pmatrix}1 & 0 & 0 & 0 \\0 & {\mathbb{e}}^{j\;\pi\;{g/8}} & 0 & 0 \\0 & 0 & {\mathbb{e}}^{2j\;\pi\;{g/8}} & 0 \\0 & 0 & 0 & {\mathbb{e}}^{3j\;\pi\;{g/8}}\end{pmatrix}} \}} & (6)\end{matrix}$

A is represented with 4 bits. Because both Λ, X represent phaseadjustments between two sets of antennas with different polarizations,they can be represented with 2 bits in total.

Through adjusting the bit numbers of D, A, Λ, X, the total number ofbits of PMI can accord to carrying capability of a transmission channel.“Accord to” as mentioned herein means that the total number of bitsrepresenting D, A, Λ, X is identical to the number of bits that thetransmission channel can transmit. For example, on a PUCCH channel, ifPMI can be transmitted in at most 4 bits, the total number of bits of D,A, Λ, X is 4. It can be provided for the UE to have a constant D=I(identity matrix), X=I. If 3 bits are set for A and 1 bit for Λ, the PMIhas total 4 bits. If the PMI is transmitted through PUSCH, the PMI mayhave 6 bits and the total number of bits of D, A, Λ, X is 6. In thiscontext, D may be represented with 1 bit and Λ may be set to I. A can berepresented with 3 bits, X in 2 bits, and thus 6 bits in total. If a7-bit PMI is transmitted on PUSCH, A can be represented with 4 bits, Dwith 1 bit, and X with 2 bits. A second codebook set is selected fromthe first codebook set. When the second codebook set is a subset of thefirst codebook set, the selecting criteria of the second codebook setfocuses on partitioning the signal space as uniform as possible. Forexample, if the first codebook set includes 7 bits in total, while thesecond codebook set only can be represented with 4 bits, we can havethat D=I, A can be represented with 3 bits, indicating 8 directions (thefirst codebook set may indicate 32 directions). In equation (5), makeg=0, g=2, G=4, 8 approximately uniform directions can be obtained. LetX=I at the same time (or Λ=I, X is represented with 1 bit), Λ isrepresented with 1 bit, with a range of values of 1 or −1 (in the caseof 2 bits, a range of 1, −1, j, −j).

Optionally, in this step, the UE can directly select a codebook setsuitable for the capability of the transmission channel to carry PMI asthe second codebook set described above according to the carryingcapability, as detailed in the following method embodiment of thisinvention.

Step 303. The UE selects a first precoding matrix for BS precodingaccording to the second codebook set.

When the UE selects precoding from the second codebook set, theselection can be made according to some specified criteria. Referencescan be made to the prior art for particular selection criteria. Forexample, a precoding matrix leading to the maximum SNR of data can beselected as the first precoding matrix.

Step 304. An index corresponding to the first precoding matrix is sentto the BS over the transmission channel.

Because the index of the selected precoding matrix needs to betransmitted over the transmission channel, different precoding matrixesin the second codebook set can correspond to different indexrespectively, so as to represent those different precoding matrixesdistinguishingly. The index corresponding to the first precoding matrixrefers to, after the index has been established for precoding matrixesin the second codebook set, an index item corresponding to the firstprecoding matrix in the established index. For example, when PUCCH isused as the transmission channel, if the first codebook set comprises 32codebook matrixes, and if PUCCH can use merely 4 bits for PMItransmission, then 16 codebook matrixes can be selected from the firstcodebook set to form the second codebook set, and 0-15 (represented with4 bits) can be used as index of the above 16 codebook matrix. If acodebook matrix with index item 0 is selected as the first precodingmatrix described above, the index item corresponding to the firstprecoding matrix can be represented as 0000; or if a codebook matrix ofnumber 2 is selected as the first precoding matrix described above, theindex item corresponding to the first precoding matrix can berepresented as 0010.

In this embodiment, after the UE acquires transmission channelcapability, a codebook set can be configured according to the channelcapability, and PMI can be feedback to the BS, so that the BS can obtainPMI with different accuracies depending on carrying capabilities of UEtransmission channels.

In another embodiment of this invention, the above method may furtherinclude:

Providing different sets of codebooks according to different thecapabilities of the transmission channels to carry PMI in advanced, theselecting action at step 302 in the above embodiment can be omitted.That is, there are several different codebook sets provided to beselected by the UE in an original state, for example, the first codebookset and the second codebook set described above. This method differsfrom that of the above embodiment in that instead of selecting thesecond codebook set from the first codebook set, the first codebook setor the second codebook set can be selected directly according to thecapabilities of the transmission channels to carry PMI.

FIG. 4 is a flowchart of a precoding method according to an embodimentof this invention, including the following steps.

Step 401. A BS receives an index corresponding to a first precodingmatrix transmitted by a UE over a transmission channel.

In this embodiment, the first precoding matrix is selected from a secondcodebook set, and the second codebook set is formed from precodingmatrixes selected from the first codebook set stored locally on the UE.A reference can be made to the corresponding description in embodimentshown in FIG. 3 for the particular way of obtaining the second codebookset.

Step 402. The BS finds out the first precoding matrix according to theindex corresponding to the first precoding matrix.

In this embodiment, the first precoding matrix can be found from a thirdcodebook set locally stored according to the index, and then precodingcan be performed on data according to the first precoding matrix,wherein the third codebook set is identical to the second codebook set.The term “identical” used herein means that the third codebook setcontains the same items of precoding matrixes as the second codebook setdoes, and the precoding matrices contained in the third codebook set arethe same as those contained in the second codebook set, and the sameindex item represents the same precoding matrix in the third codebookset and the second codebook set.

In this embodiment, the third codebook set may be predetermined by theBS according to the capability of the transmission channel to carry PMIor the type of the transmission channel. The way of establishing indexescorresponding to various precoding matrixes in the third codebook set issubstantially the same as that of the embodiment shown in FIG. 3, whichwill not be repeated herein.

Step 403. The BS performs precoding using the precoding matrix obtainedin step 403.

In this embodiment, the BS performs precoding on data sent to the UEusing the PMI which fed back from the UE. Depending on differentcarrying capabilities of UE transmission channels, precoding matrixeswith different accuracies can be obtained, so that flexible PMIconfiguration can be guaranteed and rational use of transmission channelresources can achieved.

Those skilled in the art may understand: some or all steps of the methodof the above described embodiment can be realized with programs andinstruction related hardware, the programs can be stored in a computerreadable storage medium, such as, ROM/RAM, a magnetic disk, an opticaldisc, etc.

FIG. 5 is a schematic structural diagram of a UE according to anembodiment of this invention, which can realize the method provided inthe above embodiment, the UE including:

an acquiring unit 501, configured to acquire the capability of thetransmission channel to carry PMI;

a selecting unit 502, configured to select precoding matrixes from afirst codebook set locally stored to form a second codebook setaccording to the capability of the transmission channel to carry PMIacquired by the acquiring unit 501, and then select a first precodingmatrix for performing data precoding from the second codebook set;

a transmitting unit 503, configured to carry an index corresponding tothe first precoding matrix selected by the selecting unit 502 over thetransmission channel to transmit.

Description of corresponding methods of FIGS. 3 and 4 above can bereferenced for interactions between various modules of the above UE andthe BS, and particular implementation of functions of those variousmodules.

In this embodiment, after the UE acquires transmission channelcapability, a codebook set used thereon can be configured according tothe channel capability, and PMI can be feedback to the BS, so that theBS can obtained PMI with different accuracies depending on carryingcapabilities of UE transmission channels.

FIG. 6 is a schematic structural diagram of a BS according to anembodiment of this invention, capable of realizing the method providedin the embodiment above, the BS including:

a receiving unit 601, configured to receive an index corresponding to afirst precoding matrix transmitted by a UE through a transmissionchannel, wherein the first precoding matrix is selected from a secondcodebook set by the UE, and the second codebook set is formed fromprecoding matrixes selected from a first codebook set locally stored onthe UE;

a precoding unit 602, configured to search for the first precodingmatrix in a third codebook set locally stored according to the indexreceived by the receiving unit 601, and perform precoding on dataaccording to the first precoding matrix, wherein the third codebook setis identical to the second codebook set.

In another embodiment of this invention, the BS further includes:

a transmitting unit 603, configured to transmit to the UE a signalingfor notifying the UE of the capability of the transmission channel tocarry PMI, so as to enable the UE to select precoding matrixes from thefirst codebook set to form the second codebook set. The UE and the BScan also predetermine the capabilities of the transmission channels tocarry PMI in advance.

Description of corresponding methods of FIGS. 3 and 4 above can bereferenced for interactions between various modules of the above UE andthe BS, and particular implementation of functions of those variousmodules.

In this embodiment, the BS can set up PMI feedback capabilities ofdifferent transmission channels of a UE as required, so as to obtain PMIwith different accuracies.

Note that all or some of above units can be integrated in a chip.Various functional units according to each embodiment of the presentinvention may be integrated in one processing module or may exist asseparate physical units, or two or more units may also be integrated inone unit. The integrated module may be implemented through hardware, ormay also be implemented in a form of a software functional module. Whenthe integrated module is implemented in the form of the softwarefunctional module and sold or used as a separate product, the integratedmodule may be stored in a computer readable storage medium. The storagemedium mentioned above may be ROM, a magnet disc or an optical disc.

The accompanying drawings and related description are merely for theillustration of the principle of this invention, and are not intended tolimit the scope of this invention. For example, message names andentities in various embodiments can vary with networks, and somemessages can be omitted. Any modifications, substitutions andimprovements made in the spirit and principle of this invention arecovered in the scope of this invention.

Although this invention has been illustrated and described according tosome preferred embodiments of this invention, those skilled in the artmay understand that various changes can be made in forms and details,without departing from the spirit and scope of this invention.

What is claimed is:
 1. A method for transmitting precoding matrix index(PMI), comprising: selecting a first precoding matrix from a secondcodebook set by a user equipment (UE), wherein the second codebook setis selected from a first codebook set locally stored on the UE accordingto a capability of a transmission channel to carry PMI; transmitting, bythe UE, an index item corresponding to the first precoding matrix to abase station (BS) over the transmission channel, so as to enable the BSto search for the first precoding matrix according to the index item andperform precoding on data according to the first precoding matrix. 2.The method according to claim 1, wherein that the second codebook set isa complete or a subset of the first codebook set locally stored on theUE.
 3. The method according to claim 2, wherein, when the secondcodebook set is a subset of the first codebook set, the second codebookset meets a criteria of uniformly partitioning the signal space.
 4. Themethod according to claim 2, wherein each precoding matrix in the firstcodebook set locally stored is represented as W=W₁W₂,${W_{1} = \begin{pmatrix}{DA} & 0 \\0 & {{DA}\;\Lambda}\end{pmatrix}},{W_{2} = \begin{pmatrix}1 \\X\end{pmatrix}},$ wherein, W₁ is a N_(t)×2R matrix, W₂ is a 2R×R matrix,I is a R×R identity matrix, X is a R×R diagonal matrix, D is aN_(t)/2×N_(t)/2 diagonal matrix, A is a N_(t)/2×R matrix, Λ is a R×Rdiagonal matrix, wherein N_(t) is an even and represents the number oftransmission antennas of the BS, R is the rank of the precoding matrix.5. The method according to claim 4, wherein, when the rank is 1, thesecond codebook set is a subset of the first codebook set, and D and Xare identity matrixes, the method comprises: representing Λ with 1 bithaving a value range of 1 or −1.
 6. The method according to claim 4,wherein, when the rank is 1, the second codebook set is a subset of thefirst codebook set, and D and X are identity matrixes, the methodcomprises: representing 8 uniform directions by A with 3 bits.
 7. Amethod for precoding, comprising: receiving, by a base station (BS), anindex item corresponding to a first precoding matrix transmitted by auser equipment (UE) over a transmission channel, wherein the firstprecoding matrix is selected from a second codebook set by the UE, andthe second codebook set is selected from a first codebook set locallystored on the UE according to the capability of the transmission channelto carry PMI; searching for, by the BS, the first precoding matrix in athird codebook set locally stored according to the index item, andperforming precoding on data according to the first precoding matrix,wherein the third codebook set is identical to the second codebook set.8. The method according to claim 7, wherein the second codebook set is acomplete or a subset of the first codebook set locally stored on the UE.9. The method according to claim 8, wherein, when the second codebookset is a subset of the first codebook set, the second codebook set meetsa criteria of uniformly partitioning the signal space.
 10. The methodaccording to claim 8, wherein each precoding matrix in the firstcodebook set is represented as W=W₁W₂, ${W_{1} = \begin{pmatrix}{DA} & 0 \\0 & {{DA}\;\Lambda}\end{pmatrix}},{W_{2} = \begin{pmatrix}1 \\X\end{pmatrix}},$ wherein, W₁ is a N_(t)×2R matrix, W₂ is a 2R×R matrix,I is a R×R identity matrix, X is a R×R diagonal matrix, D is aN_(t)/2×N, /2 diagonal matrix, A is a N_(t)/2×R matrix, Λ is a R×Rdiagonal matrix, wherein N_(t) is an even and represents the number oftransmission antennas of the BS, R is the rank of the precoding matrix.11. The method according to claim 10, wherein, when the rank is 1, thesecond codebook set is a subset of the first codebook set, and D and Xare identity matrixes, the method comprises: representing Λ with 1 bithaving a value range of 1 or −1.
 12. The method according to claim 10,wherein, when the rank is 1, the second codebook set is a subset of thefirst codebook set, and D and X are identity matrixes, the methodcomprises: representing 8 uniform directions by A with 3 bits.
 13. Auser equipment (UE), comprising: a selecting unit, configured to selecta first precoding matrix from a second codebook set, wherein the secondcodebook set is selected from a first codebook set locally stored on theUE according to a capability of a transmission channel to carry PMI; atransmitting unit, configured to transmit an index item corresponding tothe first precoding matrix to a base station (BS) over the transmissionchannel, so as to enable the BS to search for the first precoding matrixaccording to the index item, and perform precoding on data according tothe first precoding matrix.
 14. The UE according to claim 13, whereinthe second codebook set is a complete or a subset of the first codebookset locally stored on the UE.
 15. The UE according to claim 14, wherein,when the second codebook set is a subset of the first codebook set, thesecond codebook set meets a criteria of uniformly partitioning thesignal space.
 16. The UE according to claim 14, wherein, each precodingmatrix in the first codebook set is represented as W=W₁W₂,${W_{1} = \begin{pmatrix}{DA} & 0 \\0 & {{DA}\;\Lambda}\end{pmatrix}},{W_{2} = \begin{pmatrix}1 \\X\end{pmatrix}},$ wherein, W₁ is a N_(t)×2R matrix, W₂ is a 2R×R matrix,I is a R×R identity matrix, X is a R×R diagonal matrix, D is aN_(t)/2×N_(t)/2 diagonal matrix, A is a N_(t)/2×R matrix, Λ is a R×Rdiagonal matrix, wherein N_(t) is an even and represents the number oftransmission antennas of the BS, R is the rank of the precoding matrix.17. The UE according to claim 16, wherein, when the rank is 1, thesecond codebook set is a subset of the first codebook set, and D and Xare identity matrixes, the matrix Λ is represented with 1 bit having avalue range of 1 or −1.
 18. The UE according to claim 16, wherein, whenthe rank is 1, the second codebook set is a subset of the first codebookset, and D and X are identity matrixes, eight uniform directions arerepresented by A with 3 bits.
 19. A base station (BS), comprising: areceiving unit, configured to receive an index item corresponding to afirst precoding matrix transmitted by a user equipment (UE) over atransmission channel, wherein the first precoding matrix is selectedfrom a second codebook set by the UE, and the second codebook set isselected from a first codebook set locally stored on the UE according toa capability of the transmission channel to carry PMI; a precoding unit,configured to search for the first precoding matrix in a third codebookset locally stored by the BS according to the index item received by thereceiving unit, and perform precoding on data according to the firstprecoding matrix, wherein the third codebook set is identical to thesecond codebook set.
 20. The BS according to claim 19, wherein thesecond codebook set is a complete or a subset of the first codebook setlocally stored for the UE.
 21. The BS according to claim 20, wherein,when the second codebook set is a subset of the first codebook set, thesecond codebook set meets a criteria of uniformly partitioning thesignal space.
 22. The BS according to claim 20, wherein each precodingmatrix in the first codebook set is represented as W=W₁W₂,${W_{1} = \begin{pmatrix}{DA} & 0 \\0 & {{DA}\;\Lambda}\end{pmatrix}},{W_{2} = \begin{pmatrix}1 \\X\end{pmatrix}},$ wherein, W₁ is a N_(t)×2R matrix, W₂ is a 2R×R matrix,I is a R×R identity matrix, X is a R×R diagonal matrix, D is aN_(t)/2×N_(t)/2 diagonal matrix, A is a N_(t)/2×R matrix, Λ is a R×Rdiagonal matrix, wherein N_(t) is an even and represents the number oftransmission antennas of the BS, R is the rank of the precoding matrix.23. The BS according to claim 22, characterized in that when the rank is1, the second codebook set is a subset of the first codebook set, and Dand X are identity matrixes, the matrix Λ is represented with 1 bithaving a value range of 1 or −1.
 24. The BS according to claim 22,characterized in that when the rank is 1, the second codebook set is asubset of the first codebook set, and D and X are identity matrixes,eight uniform directions are represented by A with 3 bits.